I have the same problem with the (-1, -1) readings (consistently - not after a period of time). However in my case this appears to be the firmware not up to date. I have documented my experience so far here: https://www.gaggl.com/2017/12/grovepi-zero-connecting-your-iot-sensors/
I was hoping to use this in an educational context, but most students (and teachers for that matter) wouldn’t even get close to doing anything in the limited time of school with that many hoops to jump through.
Is there a simpler way to just update the Firmare without all of these chain of scripts?
Please note: this is meant very as constructive feedback not whinging. I would be happy to help out with forking testing a more modular approach via Gihub using standard binaries from the Raspian repos rather than cloning and compiling specific versions of dependencies on the Pi.
Also - reading the above - you state that “sudo curl -L dexterindustries.com/grovepi_setup | bash” is a quicker way compared to doing the full install via the /GrovePi/Scripts/install.sh? But from all I can see this script just loads another script which just clones the whole GrovePi repo and calls install.